This invention relates generally to magnetic resonance imaging (MRI) and spectroscopy, and more particularly the invention relates to imaging of species having short spin-spin relaxation times, T.sub.2.
Nuclear magnetic resonance (NMR) imaging, also called magnetic resonance imaging (MRI), is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is completely non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
A descriptive series of papers on NMR appeared in the June 1980 issue of the IEEE Transactions on Nuclear Science, Vol. NS-27, pp. 1220-1255. The basic concepts are described in the lead article, "Introduction to the Principles of NMR," by W. V. House, pp.1220-1226, which employ computed tomography reconstruction concepts for reconstructing cross-sectional images. A number of two- and three-dimensional imaging methods are described. Medical applications of NMR are discussed by Pykett in "NMR Imaging in Medicine," Scientific American, May 1982, pp.78-88, and by Mansfield and Morris, NMR Imaging in Biomedicine, Academic Press, 1982.
Briefly, a strong static magnetic field is employed to line up atoms whose nuclei have an odd number of protons and/or neutrons, that is, have spin angular momentum and a magnetic dipole moment. A second RF magnetic field, applied as a single pulse transverse to the first, is then used to pump energy into these nuclei, flipping them over, for example to 90.degree. or 180.degree.. After excitation the nuclei gradually return to alignment with the static field and give up the energy in the form of weak but detectable free induction decay (FID). These FID signals are used by a computer to produce images.
The excitation frequency, and the FID frequency, is defined by the Larmor relationship which states that the angular frequency, .omega..sub.o, of the precession of the nuclei is the product of the magnetic field, B.sub.o, and the so-called magnetogyric ratio, .gamma., a fundamental physical constant for each nuclear species: EQU .omega..sub.o =B.sub.o .multidot..gamma.
Accordingly, by superimposing a linear gradient field, B.sub.z =z.multidot.G.sub.z, on the static uniform field, B.sub.o, which defines the Z axis, for example, nuclei in a selected X-Y plane can be excited by proper choice of the frequency spectrum of the transverse excitation field applied along the X or Y axis. Similarly, a gradient field can be applied in the X-Y plane during detection of the FID signals to spatially localize the FID signals in the plane. The angle of nuclei spin flip in response to an RF pulse excitation is proportional to the integral of the pulse over time.
A spin echo technique has been employed in obtaining magnetic resonance signals from a body in a nonhomogeneous magnetic field. After nuclear spins are tilted and have been processed for a period of time, T, a 180.degree. refocusing RF field is applied to flip the nuclear spins 180.degree.. After a time period of T, the nuclear spins will refocus, at which time the magnetic resonance signals are detected.
However, there is an inherent delay in conventional slice-selective excitation between the peak of the RF waveform when the bulk of the transverse magnetization is created, and the end of the refocusing lobe when data acquisition can begin. Some species to be imaged have short spin-spin relaxation times (T.sub.2) after which the free induction signal significantly decays. The minimum T.sub.2 species that can be imaged is on the order of this delay time. This delay can be reduced somewhat by using offset RF waveforms and reducing the refocusing time. However the slice profile suffers.
The present invention is directed to providing a method for slice-selective excitation that allows the imaging of very sort T.sub.2 species.